Plants play a critical role as nutrients for animals, including humans, and for the production of substances useful as pharmaceuticals, cosmetics and the like. The steady growth in the world's population results in increasing needs for plant crops. This increased need must be satisfied with reduced soil resources available to agriculture. Increased crop yield can be provided with existing soil resources by engineering plant species that grow better and that are more resistant to plant pathogens.
Plants are subjected to threats by numerous pathogens, e.g., fungi, bacteria, viruses, insects and nematodes. A small fraction of pathogens succeed in invading plant tissue and thereby cause disease. In particular, potato is a major food crop that is highly susceptible to fungal infection. One strategy to combat fungal disease include the use of chemical fungicides. However, this involves high expense and environmental cost. Previous attempts to increase pathogen resistance in plants include the expression of the tomato Mi-1.2 nematode resistance gene in a nematode-susceptible tomato line. The resulting transgenic tomato plants showed resistance to the root knot nematodes M. javanica strain VW4 and M. incognita strain VW6 in most of the transgenic plants but not against M. javanica strain VW5, thus resembling the specificity of the Mi gene in wild-type plants (Milligan et al., 1998, The Plant Cell 10:1307-1319).
In another study, plant resistance was induced by activating an inactive transgene encoding the Cf-9 resistance gene product through excision of a transposable element from that gene in a plant that expressed Avr9, a Cf-9 elicitor (WO 95/31564). Also, the plant Prf resistance gene was overexpressed in tomato plants, leading to enhanced resistance to P. s. pathovar tomato strain DC3000, X c. pv. vesicatoria strain 56, R. solanacearum strain 82 bacterial pathogens and TMV viral pathogen (Oldroyd et al., 1998, Proc. Natl. Acad. Sci. USA 95:10300-10305).
In another attempt to increase plant resistance, glucose oxidase was expressed in potato plants to generate H2O2, a reagent produced during plant defense responses, through glucose oxidation. H2O2 elevation in transgenic potato plants was shown to increase resistance to E. carotovora subspecies carotovora and P. infestans (Wu et al., 1995, The Plant Cell 7:1357-1368). Transgenic rice plants were generated expressing the potato proteinase inhibitor II gene, rendering the plants more resistant to pink stem borer larvae of Sesamia inferens (Duan et al., 1996, Nature Biotechnology 14:494-498). Also, resistance to G. pallida was enhanced in transgenic potato plants expressing cowpea trypsin inhibitor (U.S. Pat. No. 5,494,813). The expression of a protein that disrupts the feeding structure of plant nematode pathogens is suggested in U.S. Pat. No. 5,866,777 and the expression of lytic proteins in apple tree plants is discussed in U.S. Pat. No. 5,824,861.
Pathogenesis-related (PR) proteins including chitinases and β-1,3-glucanases are produced by plants in response to infection by pathogens. Chitinase hydrolyzes chitin, a major component of fungal cell walls while β-1,3-glucanase hydrolyzes the glucan component (Boller et al., 1985, In: J. L. Key et al. ed., Cellular and Molecular Biology of Plant Stress, Alan R. Liss, New York, pp. 247-262; Boller, 1992, In: S. J. Gurr et al. (ed), Molecular Plant Pathology Volume II, A Practical Approach, IRL press, Oxford, pp. 23-30). Schlumbaum et al. (1986, Nature 324:365-367) have demonstrated that purified plant chitinases demonstrate antifungal activity in vitro and the expression of chitinases with one chitin-binding domain in transgenic plants enhances their resistance against fungal pathogens (Broglie et al., 1991, Science 254:1194-1197; Lin et al., 1995, Bio/Technology 13:686-691). In a resistant cultivar of Brassica napus, the mRNA of ChB4, a chitinase with one chitin binding protein, was induced by Phoma lingam infection and was detected within a day on northern blot analysis (Rasmussen et al., 1992, Plant Mol. Biol. 20:277-287). Also, chitinases act synergistically with β-1,3-glucanases in inhibiting fungal growth (Mauch et al., 1988, Plant Physiol. 88:936-942; Zhu et al., 1994, Bio/Technology 12:807-812; Jach et al., 1995, Plant J. 8:97-109)
There remains a need for plants, such as potato plants, with enhanced resistance to fungus. In particular, potato plants with enhanced resistance to soil-borne fungus.
Citation of a reference in this or in any section of the specification shall not be construed as an admission that such reference is prior art to the invention.